Technologies for deleting data stored in a data storage device have been developed in order to prevent data stored in the data storage device from leaking to a third parry due to the loss or the theft of the data storage device or an apparatus including the data storage device. When it takes time to delete all the data stored in the data storage device, the third party may abort the deletion process in the middle of processing. When half of the data of the storage capacity may be deleted, for example, the remaining half of the data will be in danger of leaking. Therefore, the user may not be reassured. To reassure a user even when a large amount of data stored in a data storage device may not be deleted, a technology has been developed, in which encrypted data is stored in the data storage device, and the decryption key is deleted in response to an instruction for deletion. Japanese Laid-open Patent Publication No. 2006-211003 discloses a related technique. With the technology, only deleting the decryption key prevents the encrypted data from easy decryption. The user, however, may still be anxious about the decryption of the encrypted data by some method.